Renal size and composition in hypertensive, obese rabbits.

OBJECTIVE

To determine whether the renal growth associated with obesity is due to hypertrophy or hyperplasia.

DESIGN

New Zealand white female rabbits were fed either standard rabbit chow (n=17) or chow fortified with 10% corn oil plus 5% lard (n=18) for 12-16 weeks.

MEASUREMENTS

All rabbits were weighed, and intra-arterial blood pressures were successfully measured at the end of the study in 16 lean and 18 obese rabbits; percent water of entire kidneys (8 lean, 8 obese rabbits) and of defined regions of kidneys (8 lean, 10 obese rabbits) were obtained gravimetrically. Renal hemoglobin, protein and DNA was measured chemically (8 lean, 8 obese rabbits).

RESULTS

Kidneys grew in size as the rabbits gained fat. In a series of 8 lean and 8 age-matched obese rabbits, weighing 3.7+/-0.1 kg and 5.4+/-0.4kg (P<0.05), the kidneys were 20% larger in the obese rabbits: 15.0+/-0.9 g vs 18.0+/-2.5 g (P<0.05). Kidney protein was also 20% greater in the obese rabbit: 1.38+/-0.06 g/kidney vs 1.66+/-0.06 g/kidney (P<0.05). While total renal DNA was 16% greater in the obese: 18.2+/-0.5 microg/kidney vs 21.1+/-0.61 g/kidney (P<0.05), no significant difference existed when the DNA was expressed as microg/mg protein. Fractional water content of the intact kidney declined with obesity: 78.7+/-1.1% vs 76.0+/-1.2% (P<0.05). Conversely, the hemoglobin content of the kidney at autopsy, an estimate of the unstressed vascular volume, increased with obesity: 55+/-19 mg/kidney vs 82+/-25 mg/kidney (P<0.05). By contrast, water content of renal parenchyma was constant: 80.8+/-1.0% vs 80.9+/-1.2% (cortex); 84.0+/-0.8% vs 83.6%+/-2.0% (outer medulla); and 85.7+/-0.8% vs 86.0+/-2.1% (inner medulla).

CONCLUSION

The renal growth associated with obesity was predominantly hyperplastic and was associated with a partial exclusion of fluid from the renal sinus.